The aim of this project is to define at the molecular level the mechanism by which trophic glycoprotein hormones (thyrotropin) elicit biological effects in their target (thyroid) cells with cholera toxin as a model system. The proposed research will focus on the study of: (i) the nature and structure of the binding sites on the B protomer of cholera toxin for its receptor the monosialoganglioside GM1; (ii) conformational changes imposed on cholera toxin and thyrotropin upon receptor interaction and (iii) mechanism of insertion and translocation into the plasma membrane. Mutual perturbations of ligand (cholera toxin, thyrotropin) and plasma membrane will be studied by spectroscopic techniques such as steady state and time resolved fluorescence spectroscopy. The potential involvement of membrane bound enzymes, NAD glycohydrolase and ADP ribosyltransferase with catalytic activities analogous to the A protomer of cholera toxin, in the intermediate phase of transmembrane signalling will be studied in the thyroid. Thyroid plasma membranes will be derived from bovine thyroid, functional (FRTL) and non functional (FRT) rat thyroid cells in culture.